Electron/infrared-phonon coupling in ABC trilayer graphene.

Stacking order plays a crucial role in determining the crystal symmetry and has significant impacts on electronic, optical, magnetic, and topological properties. Electron-phonon coupling, which is central to a wide range of intriguing quantum phenomena, is expected to be intricately connected with stacking order. Understanding the stacking order-dependent electron-phonon coupling is essential for understanding peculiar physical phenomena associated with electron-phonon coupling, such as superconductivity and charge density waves. In this study, we investigate the effect of stacking order on electron-infrared phonon coupling in graphene trilayers. By using gate-tunable Raman spectroscopy and excitation frequency-dependent near-field infrared nanoscopy, we show that rhombohedral ABC-stacked trilayer graphene has a significant electron-infrared phonon coupling strength. Our findings provide novel insights into the superconductivity and other fundamental physical properties of rhombohedral ABC-stacked trilayer graphene, and can enable nondestructive and high-throughput imaging of trilayer graphene stacking order using Raman scattering.

Catalytic Growth of Ultralong Graphene Nanoribbons on Insulating Substrates.

Graphene nanoribbons (GNRs) with widths of a few nanometers are promising candidates for future nanoelectronic applications due to their structurally tunable bandgaps, ultrahigh carrier mobilities, and exceptional stability. However, the direct growth of micrometer-long GNRs on insulating substrates, which is essential for the fabrication of nanoelectronic devices, remains an immense challenge. Here, the epitaxial growth of GNRs on an insulating hexagonal boron nitride (h-BN) substrate through nanoparticle-catalyzed chemical vapor deposition is reported. Ultranarrow GNRs with lengths of up to 10 µm are synthesized. Remarkably, the as-grown GNRs are crystallographically aligned with the h-BN substrate, forming 1D moiré superlattices. Scanning tunneling microscopy reveals an average width of 2 nm and a typical bandgap of ≈1 eV for similar GNRs grown on conducting graphite substrates. Fully atomistic computational simulations support the experimental results and reveal a competition between the formation of GNRs and carbon nanotubes during the nucleation stage, and van der Waals sliding of the GNRs on the h-BN substrate throughout the growth stage. This study provides a scalable, single-step method for growing micrometer-long narrow GNRs on insulating substrates, thus opening a route to explore the performance of high-quality GNR devices and the fundamental physics of 1D moiré superlattices.

Non-Local Electrostatic Gating Effect in Graphene Revealed by Infrared Nano-Imaging.

Electrostatic gating lies in the heart of field effect transistor (FET) devices and modern integrated circuits. To achieve efficient gate tunability, the gate electrode has to be placed very close to the conduction channel, typically a few nanometers. Remote control of a FET device through a gate electrode located far away is highly desirable, because it not only reduces the complexity of device fabrication, but also enables the design of novel devices with new functionalities. Here, a non-local electrostatic gating effect in graphene devices using scanning near-field optical microscopy (SNOM)-a technique that can probe local charge density in graphene-is reported. Remarkably, the charge density of the graphene region tens of micrometers away from a local gate can be efficiently tuned. The observed non-local gating effect is initially driven by an in-plane electric field induced by the quantum capacitance of graphene, and further largely enhanced by adsorbed polarized water molecules. This study reveals a non-local phenomenon of Dirac electrons, provides a deep understanding of in-plane screening from Dirac electrons, and paves the way for designing novel electronic devices with remote gate control.

Direct imaging of interlayer-coupled symmetric and antisymmetric plasmon modes in graphene/hBN/graphene heterostructures.

Much of the richness and variety of physics today are based on coupling phenomena where multiple interacting systems hybridize into new ones with completely distinct attributes. Recent development in building van der Waals (vdWs) heterostructures from different 2D materials provides exciting possibilities in realizing novel coupling phenomena in a designable manner. Here, with a graphene/hBN/graphene heterostructure, we report near-field infrared nano-imaging of plasmon-plasmon coupling in two vertically separated graphene layers. Emergent symmetric and anti-symmetric coupling modes are directly observed simultaneously. Coupling and decoupling processes are systematically investigated with experiment, simulation and theory. The reported interlayer plasmon-plasmon coupling could serve as an extra degree of freedom to control light propagation at the deep sub-wavelength scale with low loss and provide exciting opportunities for optical chip integration.

Patterning Graphene Films by H2O-Based Magnetic-Assisted UV Photolysis.

Properly cutting graphene into certain high-quality micro-/nanoscale structures in a cost-effective way has a critical role. Here, we report a novel approach to pattern graphene films by H2O-based magnetic-assisted ultraviolet (UV) photolysis under irradiation at 184.9 nm. By virtue of the paramagnetic characteristic, the photo-dissociated hydroxyl [OH(X2Π)] radicals are magnetized and have their oxidation capability highly enhanced through converting into an accelerated directional motion. Meanwhile, the precursor of H2O(X̃1A1) molecules distributes uniformly thanks to its weak diamagnetic characteristic, and there exists no instable diamagnetic intermediate to cause lateral oxidation. Possessing these unique traits, the H2O-based magnetic-assisted UV photolysis has the capability of making graphene microscale patterns with the linewidth down to 8.5 µm under a copper grid shadow mask. Furthermore, it is feasible to pattern graphene films into 40 nm-wide ribbons under ZnO nanowires and realize hybrid graphene/ZnO nanoribbon field-effect transistors with a hole mobility up to 7200 cm2·V-1·s-1. The X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry analyses reveal that OH(X2Π) radicals act as a strong oxidant and that another product of H(12S) adsorbs weakly on graphene.

Tunable Cherenkov Radiation of Phonon Polaritons in Silver Nanowire/Hexagonal Boron Nitride Heterostructures.

Polaritons in two-dimensional (2D) materials have shown their unique capabilities to concentrate light into deep subwavelength scales. Precise control of the excitation and propagation of 2D polaritons has remained a central challenge for future on-chip nanophotonic devices and circuits. To solve this issue, we exploit Cherenkov radiation, a classic physical phenomenon that occurs when a charged particle moves at a velocity greater than the phase velocity of light in that medium, in low-dimensional material heterostructures. Here, we report an experimental observation of Cherenkov phonon polariton wakes emitted by superluminal one-dimensional plasmon polaritons in a silver nanowire and hexagonal boron nitride heterostructure using near-field infrared nanoscopy. The observed Cherenkov radiation direction and radiation rate exhibit large tunability through varying the excitation frequency. Such tunable Cherenkov phonon polaritons provide opportunities for novel deep subwavelength-scale manipulation of light and nanoscale control of energy flow in low-dimensional material heterostructures.

Phonon Polariton-assisted Infrared Nanoimaging of Local Strain in Hexagonal Boron Nitride.

Strain plays an important role in condensed matter physics and materials science because it can strongly modify the mechanical, electrical, and optical properties of a material and even induce a structural phase transition. Strain effects are especially interesting in atomically thin two-dimensional (2D) materials, where unusually large strain can be achieved without breaking them. Measuring the strain distribution in 2D materials at the nanometer scale is therefore greatly important but is extremely challenging experimentally. Here, we use near-field infrared nanoscopy to demonstrate phonon polariton-assisted mapping and quantitative analysis of strain in atomically thin polar crystals of hexagonal boron nitride (hBN) at the nanoscale. A local strain as low as 0.01% can be detected using this method with ∼20 nm spatial resolution. Such ultrasensitive nanoscale strain imaging and analysis technique opens up opportunities for exploring unique local strain structures and strain-related physics in 2D materials. In addition, experimental evidence for local strain-induced phonon polariton reflection is also provided, which offers a new approach to manipulate light at deep subwavelength scales for nanophotonic devices.

Electrode-Free Anodic Oxidation Nanolithography of Low-Dimensional Materials.

Scanning probe lithography based on local anodic oxidation (LAO) provides a robust and general nanolithography tool for a wide range of applications. Its practical use, however, has been strongly hampered due to the requirement of a prefabricated microelectrode to conduct the driving electrical current. Here we report a novel electrode-free LAO technique, which enables in situ patterning of as-prepared low-dimensional materials and heterostructures with great flexibility and high precision. Unlike conventional LAO driven by a direct current, the electrode-free LAO is driven by a high-frequency (>10 kHz) alternating current applied through capacitive coupling, which eliminates the need of a contacting electrode and can be used even for tailoring insulating materials. Using this technique, we demonstrated flexible nanolithography of graphene, hexagonal boron nitride, and carbon nanotubes on insulating substrates with ∼10-nanometer precision. In addition, the electrode-free LAO exhibits high etching quality without oxide residues left. Such an in situ and electrode-free nanolithography with high etching quality opens up new opportunities for fabricating ultraclean nanoscale devices and heterostructures with great flexibility.